1. Field of the Invention
The present invention relates to a method of reducing nitrogen oxide content present in a gas such as a combustion exhaust gas from an engine, a boiler, an incinerator or the like, and more particularly to a method of decomposing the nitrogen oxide into nitrogen gas by means of an electric discharge into the gas containing the nitrogen oxide.
2. Description of the Related Art
As a method of reducing nitrogen oxide content by means of an electric discharge, it is known to use a corona discharger for decomposing the nitrogen oxide into nitrogen gas (e.g. the Japanese laid-open patent gazette No. Hei 2-191525). A further known method is illustrated in FIG. 8. In this method, subject gas is supplied to a gas passage 2 of a glow discharger 1' including a pair of electrodes 6, 7 opposing to each other across the gas passage 2 and a dielectric layer 4' formed on an opposing surface of at least either of the pair of electrodes 6, 7, so as to decompose the nitrogen oxide into nitrogen gas.
With either of the above-described conventional methods, decomposition of the nitrogen oxide content is to be effected by means of electric discharge into the atmosphere across the gas passage. For this reason, these conventional methods suffer problems to be described next.
In the case of a combustion exhaust gas from an engine, a typical nitrogen-oxide containing gas, the content of nitrogen oxide varies in accordance with variation in engine load. Similarly, in the case of a combustion exhaust gas from an incinerator, the content varies in accordance with the kind of objects incinerated in the incinerator. As a result, the condition between the electrodes, i.e. the insulation condition within the gas passage changes at every moment with continuous supply of the gas. Therefore, with the atmospheric discharge traversing the combustion exhaust gas flow, the discharge condition also varies in accordance with the variation in the insulation condition. Therefore, it is impossible to provide a constant amount of energy for effective decomposition of the nitrogen oxide. Consequently, decomposing performance deteriorates. Moreover, if an excessive amount of energy is supplied, the excess energy may even oxide NO (which makes up the largest portion of the nitrogen oxide present in the gas) into NO.sub.2. Further, the atmospheric discharge is effected at a great number of electrodes distributed throughout the gas passage from its entrance to exit, the gas introduced through the entrance experiences the discharging operations for a plurality of times to receive an additional amount of energy with each discharging operation. Accordingly, in the case of the combustion exhaust air which has oxygen content, the above-described oxidation reaction (NO+O) of NO into NO.sub.2 is very likely to occur.
As described above, the conventional methods can not provide good efficiency in the reduction of nitrogen oxide content by decomposing the oxide into nitrogen gas. Also, because of the problem of oxidation of NO into NO.sub.2 , a post treatment was necessary for treating this NO.sub.2. As a result, the conventional methods suffer high initial and running costs.